Method and composition for chemically polishing metals



Patented Dec. 15, 1 953 John R. Swihart, Chicago Heights, 111., assignor to The Diversey Corporation, a corporation of Illinois No Drawing. Application August 27, 1949,

- Serial-No.112,840 y 9 Claims. 1

This invention relatesto a method and com-' position for chemically polishing metals, and relates particularly to the polishing of stainless steel ces. The prior methods for polishing and brightening stainless steel surfaces include'mechanical polishing and'bumng and electropolishing. In electropolishing methods a chemical bath is used in combination with'an electric current to produce near mirror finishes on stainless steel. The use of chemical baths only without the use of electriccurrent has also been proposed, but the resulting surface has a grainy or satiny appearance as contrasted to a polished surface. Thus these prior chemical baths brighten the stainless steel surfaces but do not produce a polished effect. Electropolishing, while often producing satisfactory results, requires the use of expensive equipment and large quantities of electrical energy while mechanical polishing is diflicult because it involves extensive handling of the work resulting in high labor costs. Similarly mechanical polishing often cannot'be performed because of the intricate form and nature of the work requiring polishing. i

It is the purpose of the present invention to overcome these disadvantages and to provide a method and chemical polishing composition that produces a polished surface which nearly approaches that obtained by mechanical or electropolishing methods, yet employs only a simple chemical process, easily controlled, and utilizing commercially available materials. A further advantage of this invention is that it can be used in connection with electro or mechanical polishing as a pretreatment thereby saving valuable time, materials, and labor by reducing the amount of mechanical or electropolishing required.

No unusual or specially constructed equipment is required for this invention. Readily available acid resistant containers and heating elements are the only materials required. The invention is particularly well adapted to the removal of heat scale, dirt, and grease since it leaves the surface in polished condition usually requiring no further treatment. l

Byway of definition of terms used in the description of this polishing process, it should be 4 understood that the term "stainless steel is meant to cover those steels being mainly alloys of iron, chromium and nickel, along with minor constituents; or of only iron andchromium along with minor constituents, 1 a

In the examples and the claims, the concentrations of acids are expressed on a weight basis of the acids with the acids being of strength.

I have discovered that acid compositions composed of phosphoric, nitric, and h drochloric acids have theeiicellemperty m to chemically polish stainless steels, a property not existing in mixtures of other acids. For example, I have tried baths composed of onl hosphoric acid, hydrochloric acid an l w aE";% ydrocmacidmand water; phos horic acid, nitric acid and water; mum acid, hydrochloric acid, and water; nitri'm ac1,hy roliiEFc afiid and water; sulfuric acid, hydrofluoric acid,

0 romic acid and wager; nitric acid, hydrofluoric agid, hydrochloric acid and water; and other combinations of acids both mentrated and dilute. None of the above compositions pro duced a near mirror finish as was produced by my invention.

In the carrying out of my invention I prefer to use the new acid bath in a dilute form. While it is possible to obtain a chemical polish on stainless steels immersed in a concentrated bath of these acids, the action is very rapid and hard to control. Both time and temperature of immersion of the metal in such a bath is critical and too long an immersion will result in rapid wearing away and pitting of the metal.

The action of dilute baths on the other hand, is not so susceptible to time and temperature variations, nor does a small change in composition affect the quality of finish obtained, as for eX-. ample a slight dilution of the bath resulting when metals previously treated in some other bath, and retaining a quantity of moisture on their surfaces, are immersed in the polishing bath. Furthermore, a dilute bath is considerably less expensive than a concentrated bath.-

Accordingly I prefer to use an acid bath containing not over 30 to 35% total acidity, of which, it is essential that the m H bmwmd, or a mixture of phosphoric and s ids may be used in place 0 the phosphoric acid constituent. Active ingredients of the bath are nitric and hydrochloric acids; however, the presence of phosphoric acid or a mixture of phosphoric and sulfuric acids in substantial amounts is essential to ensure smooth chemical polishing rather than etching. When the phosphoric acid or equivalent mixture con centration of the bath is reduced to the same concentration as either the nitric or hydrochloric acids or both, an inferior polish is obtained. The lower concentration of bath which I find generally effective is one of approximately 20% total acidity.

For best chemical polishing I have found that the nitric and hydrochloric acids should each be present in at least 2% by weight and as a maximum their combined weights should not exceed the total weight of phosphoric acid. Variations between these limits have in general produced good polishes. The preferred baths, contain from about 15 to 25% by weight of phosphoric acid, 2 to 4% by weight ofnitric acid 2 to 5% by weight of hydrochloric acidfandthe remainder water. Part. or if desired all, of the phosphoric acid may be replaced with sulfuric acid. Onebath that has given excellent results'cb'nslsted essentially of about 24% phosphoric acid about 3% nitric acid, about 3% hydrochloric acid and the remainder water.

The metal to be polished is suspended in a hot acid bath of preferably not more than 210" F; temperature nor less thanlfiilg A temperature range of 180-190" F. is preferred. After considerable use the acid bath becomes depleted and. the polishing ability decreases. The bath may be replenished in several ways: (1) water losses resulting fromevaporation may be periodically replaced and the acidity of the bath improved by addition of fresh make-up acid without draining. any solution; (2) water losses and acid depletion may be periodically replaced by the addition of appropriate quantities of acid solution with. concurrent withdrawal of an amount of solution equivalent to the amount of upkeep acid added, or as another variation (3) equivalent to the amount of water and upkeep added.

When method 1 is used there is a gradual increase in the acidity of the bath, in method 2 this increase in acidity is. more gradual while in method 3 the acidity of the bath remains very uniform. Many variations and modifications of the above procedure may be employed, as for example continuous addition of upkeep acid solution and removal of used acid, without affecting the quality of polish obtained.

I may replace a portion or all of the phosphoric acid with sulfuric acid, though ordinarily the polished surface is not so bright as that obtained by using phosphoric acid without the sulfuric acid. However, if I include a small portion of an inhibitor in the acid bath to inhibit the activity of the acids, satisfactory polishes are obtained with sulfuric acid. The preferred baths, however, contain at least somev phosphoric acid. Examples of suitable inhibitors that may be used include the nitrogen containing derivatives of coal tar such as the mixture of inorganic salts of coal tar bases known as "Reilly No. 2," mixed coal tar hydrochloride bases reacted with an inorganic nitrogen compound known as Reilly No. 2, in-

hibitor in the phosphoric acid bath since its inclusion does not impair the polishing action but in some cases improves it. In general, up to 2.0% of the inhibitor may be used, but ordinarily the amount required will vary from about 0.5 to 1.0% by weight of the bath.

The time required to produce a polished surface will vary somewhat, depending upon the composition and temperature of the bath, and type of metal surface to be polished, and the condition of the surface. For normal polishing the time required will ordinarily be from about 5 to 10 minutes. At a bath temperature of F., five minutes. is. ordinarily sufiicient for most articles, and in fact such period of time is preferred as it permits better control of polishing action than is possible with shorter periods of time. -Of course, if there is a large amount of scale present, the time required may exceed 10 minutes.

The following examples are given for illustration only and it is not intended that the claims be limited to the specific concentrations of these examples.

Example 1.--A bath containing by weight of 23.9 parts phosphoric acid, 2.7 parts nitric acid, 2.2 parts hydrochloric acid, and 71.2 parts water was prepared. It was heated to and a sheet of No. 302 stainless steel was immersed for five minutes, resulting in a. bright semi-mirror finish. on the metal.

Example 2.A bath containing by weight 3.9 parts phosphoric acid, 3.0 parts nitric acid, and 2.4 parts hydrochloric acid, and 91.7 parts water was prepared, and a sheet of, No. 302 stainless steel was immersed for five minutes at 180 F. There was evidence of polish, but the results were inferior to that of Example 1.

Example 3.A similar bath to that of Example 2 was prepared except that the phosphoric acid was eliminated. The surface of a stainless steel sheet was etched, but there was no polish.

Example 4.-A bath. composed of by weight, 15.0 parts phosphoric acid, 42 parts nitric acid, 2.3 parts hydrochloric acid, and 78.5 parts water, gave a bright semi-mirror finish to a piece of No. 302 stainless steel sheet when used for five minutes at 180 F.

Example 5.-A bath composed of by weight 14.9 parts phosphoric acid, 2.8 parts nitric acid, 4.6 parts hydrochloric acid, and 77.7 parts water, gave a bright finish to a sheet of No. 302 stainless steel.

Example 6.-A portion of the phosphoric acid was replaced by sulfuric acid resulting in a bath of the following composition by weight: 3.2 parts phosphoric acid, 11.7 parts sulfuricncid, 3.5 parts nitric a cid, and 2.3'partshydrochloric acid?-1% mills No. 22 inhibitor was added, the remainder being water. The polish obtained was equally as good as in the previous example.

Excellent results have been achieved while using the new baths for the removal of scale and the chemical polishing of stainless steel generally. Particularly good results have been achieved in polishing Nos. 302, 304, 308 and 321 stainless steels. No. 302 consists of 17-19% chromium. 8-10% nickel, ODS-0.15% carbon and the remainder iron. No. 304 stainless steel consists of 18-20% chromium, 8-11% nickel, a maximum of. 0.08% carbon, 2% manganese and the remainder iron. No. 308 stainless steel consists of 19-21% chromium, 10-12% nickel, a maximum of 0.08% carbon and the remainder iron. No. 321 stainless steel consists of 17-19% chromium, 8-11% nickel, an amount of titanium that is five times the amount of carbon and the remainder iron.

Other experiments were carried out in which the Mtgaid to hydrochloric acid ratios were varied ram :1 to 1 :2, or in which the phosphoric acid was completely replaced by sulfuric acid including an inhibitor. A bright seififiniifdrsurface was obtained in all cases.

For handling and storage purposes, the phosphoric and nitric acids may be supplied in a coneentrated mixture, and then mixed with water and hydrochloric acid and an inhibitor, if any is used, to make the treating bath. This concentrated mixture preferably contains from about 60 to 75% by weight of phosphoric acid, about 8 to 12% of nitric acid, and the balance water. An excellent concentrated mixture is one containing about 65% of phosphoric acid, 9% of nitric acid and 26% of water.

The foregoing detailed description has been given for clearness of understanding only, and no unnecessary limitations should be understood therefrom.

claim:

(1.. A bath for the chemical polishing of stainless steel surfaces and the removal of scale therefrom, consisting essentially of an aqueous solution having a total acid concentration between 20 and 35% by weight and including nitric acid, hydrochloric acid, and between about 15 to 25% by weight of the total solution of an acid of the class consisting of phosphoric acid, sulfp rigacid, and a mixture of phosphoric and sulfuric acids, at least 2% of the total solution being acid and at least 2% being hydrochloric acid, with the combined weight of the nitric and hydrochloric acids being not more than the total weight of said class.

2. The bath of claim 1 wherein said member of said class is phosphoric acid.

3. The bath of claim 1 wherein said member of said class is a mixture of phosphoric and sulfuric acids.

The bath of claim 1 wherein said member of said class is sulfuric acid.

5. The bath of claim 1 wherein said bath includes an organic inhibitor.

6. A bath for the chemical polishing of stainless steel surfaces and the removal of scale therefrom, consisting essentially of about 15 to 25% by weight of phosphoric acid, about 2 to 6 4% of nitric acid, about 2 to 5% of hydrochloric acid, and the remainder being essentially water.

7. A composition of matter consisting of about 24% by weight of phosphoric acid, about 3% of nitric acid, about 3% of hydrochloric acid, and the remainder water.

8. The method of polishing a stainless steel surface which comprises contacting said surface with an acid solution consisting essentially of about 15 to 25% by weight of phosphoric acid, about 2 to 4% of nitric acid, about 2 to 5% of hydrochloric acid, and the remainder water for a period of time and at a temperature suflicient to remove scale and produce a bright polished surface.

9. The method of claim 8 wherein said period of time is from about 5 to 10 minutes and the temperature is at least about F.

JOHN R. SWIHAR'I'.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,080,348 Truitt May 11, 1937 2,166,990 Gray July 25, 1939 2,194,498 Dupernell Mar. 26, 1940 2,334,699 Faust Nov. 23, 1943 2,337,062 Page Dec. 21, 1943 2,348,359 Pray May 1, 1944 2,411,532 Escofiery Nov. 26, 1946 2,428,464 Lum Oct. 7, 1947 2,434,021 Thompson Jan. 6, 1948 2,446,060 Pray et a1 July 27, 1948 2,461,228 Miles Feb. 8, 1949 2,521,580 Hornak Sept. 5, 1950 OTHER REFERENCES Monypenny, Stainless Iron and Steel, London Chapman and Hall Publication, Cpw. 1931, pages 495 and 493.

Monypenny (Volume 1), Cpw. 1951, pages 180- 183. 

1. A BATH FOR THE CHEMICAL POLISHING OF STAINLESS STEEL SURFACES AND THE REMOVAL OF SCALE THEREFROM, CONSISTING ESSENTIALLY OF AN AQUEOUS SOLUTION HAVING A TOTAL ACID CONCENTRATION BETWEEN 20 AND 35% BY WEIGHT AND INCLUDING NITRIC ACID, HYDROCHLORIC ACID, AND BETWEEN ABOUT 15 TO 25% BY WEIGHT OF THE TOTAL SOLUTION OF AN ACID OF THE CLASS CONSISTING OF PHOSPHORIC ACID, SULFURIC ACID, AND A MIXTURE OF PHOSPHORIC AND SULFURIC ACIDS, AT LEAST 2% OF THE TOTAL SOLUTION BEING NITRIC ACIDS, AND AT LEAST 2% BEING HYDROCHLORIC ACID, WITH THE COMBINED WEIGHT OF THE NITRIC AND HYDROCHLORIC ACIDS BEING NOT MORE THAN THE TOTAL WEIGHT OF SAID CLASS. 